The invention relates to a screen cylinder for cleaning or screening fibre pulp suspension, the screen cylinder comprising screen slots and accept channels for directing the portion penetrated the screen slots to the accept side of the screen cylinder of the fibre pulp suspension fed into the feed side of the screen cylinder, and at least one feed side surface is provided between the screen slots and the accept channels comprise at least one first surface of the accept channel and at least one second surface of the accept channel.
The invention further relates to a screen wire of a screen cylinder comprising at least one feed side surface to be directed substantially in the direction of the feed side of the screen cylinder, at least one first surface to be directed in a substantially opposite direction in respect of a feed flow direction of a fibre pulp suspension and at least one second surface to be directed substantially in the same direction in respect of the feed flow direction of the fibre pulp suspension.
Screen cylinders are used for instance for cleaning and screening fibre pulp suspension. Screen cylinders can for example be manufactured by fastening parallel screen wires closely side by side in a cylindrical form such that a slot of a desired size remains between the screen wires. The screen wires form the screen or sorting surface of the screen cylinder. When employing the screen, the liquid in the fibre pulp suspension and the part of the fibres determined by the size of the slots are allowed to flow through the slots of the screen surface from the feed side or from the feed space of the screen cylinder to the accept side or to the accept space of the screen cylinder, and slivers, over-sized fibres, fibre bundles and the rest of the substance to be assorted remain on the feed side of the screen cylinder to be removed as reject from the screen. Depending on the embodiment the screen cylinder may be implemented in such a manner that the accept side of the screen cylinder is formed either on the inside or outside of the screen cylinder.
FIG. 1 schematically shows a prior art screen cylinder 1 and screen wire 2 seen from the end of the screen wire 2. FIG. 1 schematically shows three screen wires 2, which are arranged adjacent to each other at a distance from one another such that a screen slot 3 remains between them. For clarity, FIG. 1 does not show supporting wires or supporting bars, in connection with which the screen wires 2 are typically fastened. The screen wires 2 according to FIG. 1 comprise a feed side surface 4 to be directed substantially in the direction of a feed side 10 or a feed space 10 of the screen cylinder 1, a first surface 5 of an accept channel, a second surface 6 of the accept channel and an end surface 7 of the accept side connecting the first surface 5 and the second surface 6 of the accept channel. When placing screen wires 2 next to one another an accept channel 8 is thus formed between the first surface 5 and the second surface 6 of the accept channel, the accept channel extending from the screen slot 3 to an accept side 9 or an accept space 9. The feed side surfaces 4 of the screen wires 2 thus form together a screen surface 16 or a sorting surface 16 of the screen cylinder provided with screen slots 3 between the screen wires 2. In the position of the screen cylinder 1 shown in FIG. 1 the feed side 10 or the feed space 10 of the screen cylinder is found above the screen wires 2 and the accept side 9 or the accept space 9 of the screen cylinder 1 is found below the screen wires 2.
In the screen cylinder 1 according to FIG. 1 the feed side surface 4 of the screen wires 2 is formed to be oblique or slanting in relation to the tangent of the screen cylinder 1 such that when screen wires 2 are placed adjacent to one another a step is formed between the surfaces 4 of the feed side 10 in the feed direction of the fibre pulp suspension shown by arrow A in such a manner that a back part 4′ of the feed side surface 4 of the previous screen wire 2 in the feed direction of the fibre pulp suspension is placed higher than the front part 4″ of the feed side surface 4 of the latter screen wire 2 in the feed direction of the fibre pulp suspension. The aim of the profile of the feed side surface 4 of such a screen wire 2 is to provide a clockwise rotating turbulent whirl indicated by arrow B, as a consequence of which the bonds between the fibres are unravelled. After this the fibre pulp suspension flows into a screen slot 3, which forms a mechanical obstacle for the large particles in the fibre pulp suspension. After the screen slot 3 the cross-sectional area of the accept channel 8 increases and the flow rate of the fibre pulp suspension slows down before directing the flow into the accept space 9.
The whirl indicated by arrow B formed on the feed side 10 therefore reverts the flow direction of the fibre pulp suspension before the screen slot 3 to the opposite direction in relation to the flow direction of the fibre pulp suspension indicted by arrow A on the screen surface 16. As a consequence the fibre pulp suspension aims to continue the flow after having penetrated the screen slot 3 along the second surface 6 of the accept channel of the screen wire 2, i.e. as shown in FIG. 1 along the left-side edge of the accept channel 8 as shown by arrows C, whereby an area of slower flow rate is formed on the right side of the accept channel 8 as shown in FIG. 1, in which area a back flow whirl indicated by arrow D is easily created in the fibre pulp suspension that moves more slowly, which may cause the fibres to spin and the pressure loss of the screen to increase. What is further easily formed on the feed side surface of the screen wire 2 is a stagnation point 11 gathering impurities that cannot be cleaned with the rinsing flows of the screen wires 2 indicated by arrows E and directed backwards along the accept channel 8, since the rinsing flows are not directed in the same direction as the screen surface when being discharged from the screen slot 3 to the feed side 10.
Publication U.S. Pat. No. 6,273,266 shows a solution for keeping the screen surface clean by means of the rinsing flows of the screen. In the solution shown in the publication an extension is formed on the feed side of the screen wire that extends above the screen slot and further partly above the feed side surface of the following screen wire in the feed direction of the fibre pulp suspension. In accordance with the solution, the extension is implemented such that an angle, the size of which ranging from 3 to 45 degrees and preferably being 5 to 25 degrees, is formed between the lower surface of the extension and the feed side surface of the screen wire. Such an extension allows directing the rinsing flow arriving on the feed side from the accept channel during the rinsing flow along the feed side surface of the screen wire in the direction of rotation of feed side flow. In addition, the clockwise rotating whirl formed during the flow-through may be converted to a counter-clockwise rotating whirl. Together the rinsing flow along the feed side surface of the screen wire and the counter-clockwise rotating whirl rinse a layer formed during the flow-through period in the stagnation point on the feed side surface of the screen wire, thus keeping the screen cleaner than before. However, a problem with the screen according to publication U.S. Pat. No. 6,273,266 is still that a part of the fibre pulp suspension moving more slowly is formed on the accept channel, where the previously described back flow whirl can easily be formed, which may cause the fibres to spin and the pressure losses of the screen to increase.